@TP-Link 

Thanks for the advice about splitting the network with 5GHz for main network and 2.4GHz for guest network. That was a key to solving my issues as having multiple networks using the same frequencies caused interference, especially when the additional APs in the mesh network were also using these same frequencies for backhaul. These are things that I did to fix the problems:

1. Switched main network to 5GHz only

2. Switched guest network to 2.4GHz only

3. Put all IoT (smart home) devices on guest network

4. Configured guest network to use ONLY the following security encryption protocol: WPA2-PSK [AES].  No other protocol would work and allow all devices to connect. This is the only one which worked and finally switching to this one allowed everything to connect.

5. Connected 2nd and 3rd access point to main access point using ethernet backhaul. This required a bunch of reconfiguring of my network and running a new ethernet cable. The instructions for this are in another post in this forum. Check it out here: https://community.tp-link.com/en/home/forum/topic/98898

After doing all of those things, my network is stable with no device drop-offs. It was WAY harder than it should have been, but that's not really TP-Link's fault. The information in these forums is great but when you're dealing with a bunch of IoT devices, there are MANY variables to work out.

Hopefully this info helps others who might be having the same issues that I did.